TECHNICAL ARTICLE
ACCURATELY CAPTURE TRUE
ACCELERATION DATA IN ON-VEHICLE
DVR SYSTEMS USING A MEMS
ACCELEROMETER AND THE ADAPTIVE
REFERENCE METHOD
Ben Wang
Applications Engineer,
Analog Devices, Inc.
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Modern on-vehicle digital video recording systems (DVRs) or OBDs are
using accelerometers (g-sensors) to measure on-vehicle acceleration.
This allows the DVR to watermark the recording video with date/time/
acceleration information when there are predefined events that occur,
such as hard braking or a collision. The watermark is helpful when
saving the video to system memory such as a hard disk or SD card.
The watermark makes it easy to identify and play back the event video of
interest. And the system memory can be saved significantly by keeping
watermarked video only and deleting others. However, it is a big challenge
to accurately measure acceleration when the vehicle is running due to the
combined Earth gravity offset and vehicle vibration on the accelerometer.
This article introduces a simple but effective way to solve this problem.
Z
g Value Generated by Hard Braking
–2 g
V
1s
20 ms
Vehicle Speed Reduced to 0 ms from 20 ms
G-Sensor
(ADXL313)
0
t
Figure 2. Acceleration and speed vs. time when vehicle is hard braking.
I2C INT
CMOS
Sensor
t
Processor
SD
Memory
Figure 1. On-vehicle DVR system block diagram.
Figure 1 shows an on-vehicle DVR system block diagram. Camera video
from the CMOS sensor is taken, processed, and ultimately stored in
standalone memory, for example, SD card or a hard disk. As highlighted
in blue, an accelerometer, for example an ADXL313, is used to measure
vehicle acceleration.
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Figure 2 explains how a DVR system with accelerometer works. In the
case of a predefined event, for example, hard braking, acceleration is
increasing or decreasing significantly as the vehicle’s speed varies. The
accelerometer will sense and measure this acceleration which the data
MCU/processor can capture and process. Once the acceleration crosses
a predefined threshold (for example, –1.5 g), the DVR system will start to
watermark the recording video with information such as the date/time/
acceleration value.
In reality, the measured acceleration from the accelerometer is not an
accurate reflection of the real vehicle acceleration due to distortion caused
by Earth’s gravity offset and vehicle vibration. There are many cases in
which Earth’s gravity offset is introduced. For example, when the DVR is
installed in the rear view mirror, the mirror’s surface angle with Earth’s
gravity is uncertain since the passenger can adjust it by hand. Another
example is when the vehicle is traveling on a road that is not 100%
horizontal. Also, vibrations from the vehicle’s engine and rugged road
conditions are randomly coupled into the acceleration measurement and
cause errors.
Accurately Capture True Acceleration Data in On-Vehicle DVR Systems
Using a MEMS Accelerometer and the Adaptive Reference Method
Table 1. Earth Gravity Offset Introduces Acceleration
Measurement Error
Road Slope vs. Induced Gravity Offset
Gravity Offset on
Z-Axis (g)
0
0.017452406
0.034899497)
0.052335956
0.069756474
0.087155743
0.104528463
0.121869343
0.139173101
0.156434465
0.173648178
0.190808995
0.207911691
0.224951054
0.241921896
0.258819045
Road Slope Ө(˚)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Real Vehicle g
on Z-Axis (g)
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Sensor Measured g on
Z-Axis (g)
1
1.017452406
1.034899497
1.052335956
1.069756474
1.087155743
1.104528463
1.121869343
1.139173101
1.156434465
1.173648178
1.190808995
1.207911691
1.224951054
1.241921896
1.258819045
Table 1 is an examination of how significant error can be introduced by
Earth’s gravity offset. The first column is the angle of road slope related to
the horizon; the second column is the offset on the g-sensor z-axis that
is introduced by Earth’s gravity; and the fourth column is the sensormeasured acceleration on the z-axis. With 1 g of acceleration on a vehicle,
the measured acceleration on the z-axis increases with the angle, as
the fourth column shows. For example, at an angle of 15° the measured
acceleration is about 1.26 g with 1 g real acceleration on z-axis, so the
measured error is about 26%.
X-g
2.5
B
2.0
C
1.5
~2 g
1.0
~1.25 g
~1.5 g
0.5
example clearly demonstrates that gravity offset will introduce errors on
measured acceleration, which will cause the system to make incorrect
decisions. And in reality both gravity offset and vibration are unpredictable,
so the error introduced on measurement is also unpredictable. It is quite a
challenge to extract real acceleration from the measured data respective
of gravity offset and vibration.
However, adaptive reference can adaptively eliminate the error introduced
by gravity offset and vibration. It instantly monitors the data from the
accelerometer, and any data that exhibits small and slow variation over
time is considered to be error introduced by gravity offset and vibration.
This is highlighted by the yellow line in Figure 3. The real acceleration
of interest exhibits large and fast variation over time and can be identified
and extracted from the measured data by eliminating the error introduced
by gravity offset and vibration. This is highlighted as pulses A, B, and C in
Figure 3. The basic operation to achieve real acceleration data is described
in the following paragraph.
Upon every measurement cycle, the accelerometer measures and stores
the data which is used as a reference for the next cycle calculation. At
the next measurement cycle, the measured data will be compared to the
reference data from the previous cycle, and the results data is compared
to a predefined threshold. If the results data is over a predefined threshold,
it is identified as a large and fast transit of interest, and will be further
processed by system software. If the results data does not exceed the
threshold, it is identified as offset and noise introduced by gravity and
vibration. The measurement cycle requires a fine tuning algorithm to
achieve accurate detection based on different environments. The above
process can be expressed with the following formula:
ABS [gn – gn – 1] > gth
Where
gn = g data measured in current measurement cycle
gn – 1 = g reference measured in previous cycle
gth = predefined g threshold
2.5
A
B
2.0
A
1.5
0
1.0
−0.5
0.5
C
~2 g
~1.5 g
~1.25 g
0
Gravity Offset Change
−0.5
793
757
721
685
649
613
577
541
505
469
433
397
361
325
289
253
217
181
145
829
847
811
775
739
703
667
631
595
559
523
487
451
415
379
343
307
271
235
199
163
73
109
91
127
Figure 3 shows a real acceleration event measured with Earth’s gravity
offset and vehicle vibration coupled onto the accelerometer. In the diagram
the blue line represents acceleration as measured on the accelerometer,
and the yellow line depicts the gravity offset introduced by Earth’s gravity.
As the diagram shows, there are three peak points observed, points
A, B, and C. Point A is about 1.25 g measured, Point B is about 2.25 g
measured, and Point C is about 1.75 g measured. By predefining the
threshold at 1.5 g, both points B and C are over the threshold, while
Point A is below threshold. But, in fact, this result is wrong since gravity
offset on the accelerometer is not compensated. As the red color highlighted
in the diagram shows, by eliminating the gravity offset effect the real
acceleration of Point A is about 1.5 g, Point B is about 2 g, and Point C is
about 1.25 g. In this case with the predefined threshold of 1.5 g, Point A
and Point B are over the threshold, and Point C is below threshold. This
−2.5
1
Figure 3. Acceleration output coupled with Earth’s gravity offset and vehicle vibration.
−2.0
19
793
757
721
685
649
613
577
541
505
469
433
397
361
325
289
253
217
181
145
829
847
811
775
739
703
667
631
595
559
523
487
451
415
379
343
307
271
235
199
163
73
91
127
109
−1.5
1
−1.0
−2.5
37
−2.0
37
−1.5
55
−1.0
55
2
19
Figure 4. Accurate acceleration after the adaptive reference method is applied.
Figure 4 depicts the calculated real acceleration by eliminating the error
introduced by gravity offset and vibration. As shown in the diagram,
now the yellow line is close to zero, which means that gravity offset
and vibration are almost eliminated. And points A, B, and C are correctly
reflecting the real acceleration.
Generally speaking, the adaptive reference method described above
should be implemented by software, but in reality it may not be practical
to expect the MCU or processor to accomplish it with pure software since
the video application is real time and the MCU or processor may lack
the resources. As a solution, the ADI ADXL313W accelerometer features
ac mode operation and a built-in 32 deep FIFO, which greatly helps
Visit analog.com in implementing the adaptive reference method to achieve accurate
acceleration measurements even with a resource constrained back-end
MCU or processor. AC mode operation allows the ADXL313W to keep
measured data as the reference used for the next cycle calculation and
the built-in 32 FIFO enables the ADXL313W to preserve up to 32 words
of measured data, both of these significantly off-load the back-end MCU
or processor.
Configure ADXL313
in AC Mode and
Set Up Threshold
Video
Recording
ADXL313
INT Active?
The modern vehicle DVR or OBD requires accurate acceleration detection
and measurement in order to record watermarked video of interest with
a limited memory size. Measurement errors are mainly introduced by
Earth’s gravity offset and vehicle vibration, and they are unpredictable,
which presents a design challenge to the system designer. The adaptive
reference method can be implemented by software to eliminate the
error, but this, in reality, may not be practical since DVR or OBD systems
can easily be resource constrained. The ADI ADXL313W accelerometer
features both ac mode operation and a 32 deep FIFO, which greatly helps
with implementing the adaptive reference method, while significantly offloading the back-end MCU or processor. Combined with more features
such as automotive qualified, high resolution, low noise, and low power,
the ADXL313W helps to greatly improve DVR system performance.
About the Author
Y
Watermarking
Video
N
Conclusion
Delay
X ms
Ben Wang [ben.wang@analog.com] joined ADI as a field applications
engineer in 2009, located in Shenzhen, China. Prior to joining
Analog Devices, Ben worked for six years at National Semiconductor.
Ben graduated from Hunan University, China.
Figure 5. Flow chart to utilize the ADXL313W’s ac mode for accurate acceleration
calculation.
Online Support
Community
Figure 5 is a flow chart of ADXL313W ac mode operation. Once ac mode
operation is activated, the ADXL313W will automatically keep the previously measured data as the reference for next cycle comparison against
the predefined threshold; if it is over the threshold, an interrupt signal will
be active to inform the MCU or processor to deal with it. In the flow chart,
delay X ms is set as the interval time between two measurement cycles,
and can be fine-tuned subject to the application.
Engage with the Analog Devices
technology experts in our online
support community. Ask your
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Configure ADXL313
in FIFO Mode and
INT Enabled
Video
Recording
N
ADXL313
INT Active?
Watermarking
Video
Access ADXL313
for New g Data
Running ABS (g – gf)
and Updating gf
Y
N
ABS [g – gf] > gth
Figure 6. Flow chart to utilize the ADXL313W FIFO for accurate acceleration calculation.
Figure 6 is a flow chart of ADXL313W 32 FIFO operation. Once 32 FIFO
mode operation is enabled, the ADXL313W will automatically keep up to
32 words of data in the FIFO, and if the FIFO is full, an interrupt signal
goes active to inform the MCU or processor accordingly.
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